Method for controlling an internal combustion engine equipped with a device for recycling exhaust gases

ABSTRACT

Process for controlling an internal combustion engine equipped with an exhaust gas recirculation device ( 5 ) comprising a recirculation valve ( 51 ) and a device for regulating an air/fuel mixture injected into an inlet circuit as a function of the signal (LAM_AV) of an oxygen probe ( 41 ) placed in the exhaust circuit ( 4 ), characterized in that the controls of the valve (EGR_CTRL) are synchronized with at least one transition of a correction (LAM_COR) of the richness of the mixture in order to reduce the transient pollution peaks generated by the operating of the exhaust gas recirculation valve ( 51 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for controlling an internalcombustion engine, and relates more particularly to such a processdesigned to control the recirculation of exhaust gases.

Because of the requirements to reduce the pollution of internalcombustion engines, exhaust gases have to be recirculated to the inletside of the engine in certain operating regimes.

2. Description of the Related Art

Document FR 2 677 123, for example, discloses a process for controllingand diagnosing an internal combustion engine comprising an exhaust gasrecirculation device. The engine is controlled conventionally bycontrolling an amount of fuel delivered by an injector placed in theinlet manifold to suit the operating parameters such as the pressure inthe inlet manifold and the engine speed. An oxygen probe placed in theexhaust manifold allows the richness of the mixture to be regulated tokeep it around the stochiometric value. The engine further comprises anexhaust gas recirculation device consisting of a recirculation pipeconnecting the exhaust manifold to the inlet manifold, said pipe beingequipped with a valve that allows the amount of exhaust gasesreintroduced in the inlet side to be regulated.

In certain operating regimes in which it is necessary to obtain areduction in the temperature of combustion in the engine cylinder, therecirculation valve is therefore made to open so that neutral gas can beintroduced into the mixture. In this way it becomes possible to reducethe omissions of polluting gases, such as oxides of nitrogen forexample, in the exhaust gases.

Document JP-A-54 124 119 suggests synchronizing the control of therecirculation valve with the transitions of a signal from an oxygenprobe placed in the exhaust manifold. However, the intended objective isto minimize variations in torque and does not take account of thepollution generated by synchronization of this kind.

It has been found that when the recirculation valve is made to open orto close, a peak of pollutants liable to exceed the purificationcapability of a catalytic converter placed in the exhaust manifoldoccurred.

The object of the present invention is therefore to provide a processfor controlling an internal combustion engine which is more particularlysuited to reducing these pollution peaks. The objects of the inventionare achieved by means of a process for controlling an internalcombustion engine equipped with an exhaust gas recirculation devicecomprising a recirculation valve and with a device for regulating theair-fuel mixture injected into the inlet circuit as a function of thesignal from an oxygen probe placed in the exhaust circuit. According tothe invention, control of the recirculation valve is synchronized withat least one transition of a correction of the richness of the mixtureso that opening and/or closure of the recirculation valve occurrespectively during a phase in which the air/fuel mixture is beingenriched and/or a phase during which it is becoming more lean.

According to the invention, the control of the recirculation valve issynchronized with at least one transition of a correction of therichness of the mixture.

In a first embodiment of the invention, the valve is made to open in afirst time window, the start and the end of which are determined suchthat the actual opening of the valve, which is defined by an increase inthe inlet pressure at the inlet to a cylinder of the engine as a resultof the opening of the valve, occurs during a phase in which the air/fuelmixture is enriched. Advantageously, this first time window for allowingthe valve to be opened is determined by two durations measured from atransition of the mixture from rich to lean, these durations being afunction of the speed and of a value representing the load of theengine.

Likewise, the valve is made to close in a second time window, the startand end of which are determined such that the actual closure of thevalve, which is defined by a decrease in the inlet pressure at the inletto a cylinder of the engine as a result of the closure of the valve,occurs during a phase in which the air/fuel mixture becomes more lean.Advantageously, this second time window for allowing the valve to beclosed is determined by two durations measured from a transition of themixture from lean to rich, these durations being a function of the speedand of a value representing the load of the engine.

In a second embodiment, the time windows begin respectively upon atransition from lean to rich, in the case of opening, and from rich tolean, in the case of closing, and end after a fifth, or respectivelysixth, duration, said durations being a function of the speed and of avalue representing the load of the engine.

SUMMARY OF THE INVENTION

Other features and advantages of the process according to the inventionwill become clear on reading the description which will follow and onexamining the appended drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an internal combustion engine equipped with an exhaustgas recirculation device controlled by a computer capable ofimplementing the process according to the invention,

FIG. 2 depicts the functional architecture of said computer, and

FIG. 3 depicts timing charts of signals allowing the way in which theinvention works to be explained.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to FIG. 1 which depicts an internal combustionengine comprising an inlet manifold 1, an engine block 2 and an exhaustmanifold 4. The inlet manifold comprises a throttle valve 10, possiblyequipped with an angular-position sensor 11 delivering a signal THRrepresenting the position of the throttle valve. An air mass flow ratesensor 12 provides a signal MAF representing the mass of air enteringthe inlet manifold. The manifold also comprises a cavity 14 known as aplenum in which there is placed an absolute-pressure sensor 13delivering a signal MAP. The engine block 2 is equipped with arotational-speed sensor 21 delivering a speed signal N. The exhaustmanifold 4 contains a catalytic converter 40 intended to purify theexhaust gases and an oxygen probe 41 delivering a signal LAM_AVrepresenting the oxygen content of the exhaust gases and therefore therichness of the mixture burnt in the engine.

The engine is also equipped with an exhaust gas recirculation device 5comprising a pipe 50 placing the exhaust manifold 4 in communicationwith the inlet manifold 1 upstream of the plenum 14. A recirculationvalve 51 is placed in the pipe to control the amount of exhaust gasrecirculated to the inlet side. Also depicted is a computer 6 receivingsignals from the various sensors MAF/MAP, LAM_AV, N, THR and othersignals such as the temperature of the air entering the engine, thetemperature of the coolant, from sensors, not depicted. The computeralso supplies a signal TI for controlling an injector 3 allowing theamount of fuel supplied to the engine to be metered, together with asignal EGR_CTRL for controlling the exhaust gas recirculation valve.

It has been found that the pollution peaks observed when operating therecirculation valve arose from the effect of a pressure wave obtainedupon opening or closing the valve 51 in the pipe 50 and in the plenum14. Specifically, when the valve is opened, the exhaust gases present inthe manifold 4 push a column of air along the pipe 50 and this causes anincrease in the partial pressure of oxygen at the inlet to the cylinderof the engine 2. Likewise, when the valve 51 is closed, a transientdepression is observed in the pipe 50 and this results in a drop in thepartial pressure of oxygen at the inlet to the cylinder. These variantsin oxygen pressure in the cylinder lead to variations in the richness ofthe burnt mixture and therefore to peaks of pollutants liable not to bepurified by the catalytic converter 40.

The invention therefore consists in synchronizing the effect of thesevariations in oxygen pressure with phases of enriching the supply offuel or, respectively, of making it more lean, the supply beinggenerated by closed-loop control of the richness of the mixtureperformed by the oxygen probe 41, as will be seen later in conjunctionwith the graphs of FIG. 3.

Reference is now made to FIG. 2 which depicts the partial functionalarchitecture of the computer 6 for those parts which are relevant to theprocess according to the invention. A first block B1 receivesinformation from the inlet-pressure sensor MAP and/or from the air massflow rate sensor MAF, the speed N, the degree of openness of thethrottle valve THR, and other information not depicted such as the airtemperature and/or the coolant temperature. It formulates a basicinjection time TIB from a map obtained during engine bench tests. Asecond block B2 receives the signal LAM_AV from the oxygen probe 41 anda richness setpoint value LAM_SP and produces a richness correctionsignal LAM_COR. The two signals are then transmitted to a block B3 whichin the conventional way produces the signal TI for controlling theinjector 3. A fourth block B4 receives the signal N representing therotational speed of the engine and a signal representing the engineload. This signal may be deduced from the pressure MAP in the inletmanifold or alternatively from the inlet air mass flow rate MAF and fromthe speed N or alternatively from the position of the throttle valve THRand from the speed. In the example which follows, we shall adopt theinlet pressure MAP as being the variable representing the engine load.This block B4, from these signals, formulates four durations T1 and T4from a table stored in memory during engine bench testing. The method inwhich these durations are formulated will be explained later inconjunction with FIG. 3. A fifth block B5 also receives signalsrepresenting the engine load and the speed to produce an exhaust gasrecirculation setpoint signal EGR_SP. The durations T1 to T4, thesetpoint signal EGR_SP and the richness correction signal LAM_COR aresupplied to a block B6 to produce a signal EGR_CTRL for controlling thevalve 51.

Reference is now made to FIG. 3 to detail the control process accordingto the invention. FIG. 3(a) depicts a time graph of the signal LAM_AVdelivered by the richness probe 41. This signal has two states, denotedR and P respectively, and which represent the combustion of a richmixture and that of a lean mixture. A signal of this kind is obtained,for example, by means of an oxygen probe of the titanium oxide TiO₂ orzirconium oxide ZrO₂ type.

The richness correction signal LAM_COR produced by the block B2 andintended to correct the basic injection time TIB in the block B3 isdepicted in (b) in the same phase. As can been seen in the figure, whenthe signal from the probe 41 represents combustion of a lean mixture,the correction signal LAM_COR is used to enrich the mixture by aproportional transition followed by an integral ramp until the probesignal changes state. The mixture is thus alternately enriched and mademore lean in phases, depending on the state of the probe.

The exhaust gas recirculation setpoint signal EGR_SP from block B5 isdepicted in (c). Depending on the engine load and speed conditionsprevailing at the input to the block B5, this signal exhibits an initialstate DES_C, for example representing an “absence of recirculation”(valve closed) setpoint. At the instant t1, it is assumed that the speedand load conditions are such that exhaust gas recirculation may beenvisaged. The setpoint signal EGR_SP therefore adopts a value DES_0representing an open setpoint. Of course, these values could just aseasily be open setpoint values other than “wide open” or “fully closed”.It is, for example, possible to envisage the signal quantitativelyrepresenting a desired degree of opening of the valve. At the instantt2, as the exhaust gas recirculation conditions are no longer allfulfilled, the setpoint signal returns to a value DES_C corresponding tothe valve being closed.

We now come, in conjunction with graph (d), to the operation of blockB6. At the instant t3 corresponding to a transition of the signalLAM_COR beginning a phase in which the mixture becomes more lean, theblock B6 counts down the durations T1 and T2 thus defining, from thestart of the becoming-lean phase, a first time window Z1. As soon asthis time window opens, in the presence of a recirculation setpointsignal with the state DES_0, the block B6 switches the signal EGR_CTRLfor controlling the valve 51 from the state CTRL_C (closed) to the stateCRTL_O (open). Likewise, at the instant t5 corresponding to the start ofa mixture-enriching phase, the block B6 counts down the durations T3 andT4 defining a second time window Z2. At the instant T2 which fallsinside the time window Z2, with the setpoint signal EGR_SP switching tothe state DES_C, the signal EGR_CTRL for controlling the valve alsoswitches to the state CTRL_C, representing closure of the valve 51.

As was seen earlier, the opening of the valve 51 sets up a pressure wavein the pipe 50 and in the plenum 14, generating a transient overpressureof oxygen at the inlet to the engine cylinder. As can be seen in graph(e) of FIG. 3, where the partial pressure of oxygen is depicted on theordinates and time is depicted on the abscissa, upon opening of thevalve at the instant t4, the partial pressure of oxygen increases. Thistransient increase has the effect of causing lean-mixture combustionwhich has to be wiped out by synchronizing the pressure peak with atransition of the correction of the air/fuel mixture toward a richmixture. However, bearing in mind parameters such as the geometry of theinlet circuit and the dynamics of the gases, the effect of opening thevalve is not felt immediately at the inlet to the cylinder. The actualopening of the valve is therefore determined by the instant at which theoxygen overpressure reaches a predetermined threshold ΔP₁. The durationT1 is thus defined in such a way that an opening of the valve, occurringimmediately after T1, does not have an effect in the cylinder untilafter a transition from lean to rich of the signal LAM_COR determiningthe start of a mixture-enriching phase. Likewise, the duration T2 isdefined in such a way that an opening of the valve 51, occurring at theend of the duration T2, leads to an effective opening of the valveoccurring before the end of this air/fuel mixture enriching phase. Thedurations T1 and T2 are determined in a table as a function of the speedand load perimeters by engine bench tests, during engine development.The durations T3 and T4 for the closing of the valve are determined in asimilar way, these symmetrically causing a transient drop in the partialpressure of oxygen, so that closure of the valve inside the second timewindow Z2 produces its effect during the next phase of making themixture more lean. Time windows Z1 for opening (and Z2 for closing) arethus generated upon each transition of the richness correction signalLAM_COR from rich to lean (or from lean to rich). A command to open orto close the valve 51 is produced during these time windows only if thesetpoint signal EGR_SP has changed state immediately before or duringthis window.

A second embodiment of the invention will now be described inconjunction with graph (f) of FIG. 3. In this embodiment, the start ofthe first time window Z′1 is determined by a transition of the air/fuelmixture from lean to rich. The end of the time window Z′1 is obtainedafter a fifth duration T5 such that the effect of opening the valve isfelt during the enriching phase beginning at this transition. Likewise,the start of the second time window Z′2 is synchronized with atransition of the mixture from rich to lean at the instant t7. Thissecond time window Z′2 ends at the end of a duration T6 calculated suchthat the effect of closing the valve 51 at the end of the window remainsinside the same phase of the mixture becoming more lean. This embodimentof the invention has the advantage, over the previous embodiment, ofrequiring less calculation power and memory capacity to implement it.

Of course, the invention is not restricted to the embodiment described,but could alternatively be achieved by determining the durations T1 toT4 and T5 and T6 as a function of the period of time between twoconsecutive transitions of the mixture correction signal from rich tolean or from lean to rich.

What is claimed is:
 1. A process for controlling an internal combustionengine with an exhaust circuit, an oxygen probe in the exhaust circuit,and an exhaust gas recirculation device with a recirculation valve and adevice for regulating an air/fuel mixture injection into an inletcircuit as a function of a signal of the oxygen probe, the method whichcomprises: synchronizing a control of the recirculation valve with atransition of a correction of a richness of the air/fuel mixture;opening the recirculation valve in a first time window, and defining astart and an end of the first time window such that an actual opening ofthe valve, defined by an increase above a predetermined threshold in aninlet pressure at an inlet to an engine cylinder as a result of theopening of the valve, occurs during a phase wherein the air/fuel mixtureis enriched; and closing the recirculation valve during a phase whereinthe air/fuel mixture becomes more lean.
 2. The process according toclaim 1, which comprises: defining the start of the first time window atthe end of a first time period after a transition of the air/fuelmixture from rich to lean; defining the end of the first time window atthe end of a second time period after the transition of the air/fuelmixture from rich to lean; and defining the first time period and thesecond time period as a function of a speed of the engine and a valuerepresenting a load of the engine.
 3. The process according to claim 1,which comprises: defining the start of the first time window upon atransition of the air/fuel mixture from lean to rich; defining the endof the first time window at an end of a fifth time period after thetransition of the air/fuel mixture from lean to rich; and defining thefifth time period as a function of a speed of the engine and of a valuerepresenting a load of the engine.
 4. The process according to claim 1,which comprises closing the recirculation valve in a second time window,and defining a start and an end of the second time window such that anactual closing of the valve, defined by a decrease above a predeterminedthreshold in an inlet pressure at an inlet to an engine cylinder as aresult of the closure of the valve, occurs during a phase wherein theair/fuel mixture becomes more lean.
 5. The process according to claim 4,which comprises: defining the start of the second time window at the endof a third time period after a transition of the air/fuel mixture fromlean to rich; defining the end of the second time window at the end of afourth time period after the transition of the air/fuel mixture fromlean to rich; and defining the third time period and the fourth timeperiod as a function of a speed of the engine and a value representing aload of the engine.
 6. The process according to claim 4, wherein:defining the start of the second time window upon a transition of theair/fuel mixture from rich to lean; defining the end of the second timewindow at an end of a sixth time period after the transition of theair/fuel mixture from rich to lean; and defining the sixth time periodas a function of a speed of the engine and of a value representing aload of the engine.
 7. A process for controlling an internal combustionengine with an exhaust circuit, an oxygen probe in the exhaust circuit,and an exhaust gas recirculation device with a recirculation valve and adevice for regulating an air/fuel mixture injection into an inletcircuit as a function of a signal of the oxygen probe, the method whichcomprises: synchronizing a control of the recirculation valve with atransition of a correction of a richness of the air/fuel mixture;opening the recirculation valve during a phase wherein the air/fuelmixture is enriched; and closing the recirculation valve in a definedtime window, and defining a start and an end of the defined time windowsuch that an actual closing of the valve, defined by a decrease above apredetermined threshold in an inlet pressure at an inlet to an enginecylinder as a result of the closure of the valve, occurs during a phasewherein the air/fuel mixture becomes more lean.
 8. The process accordingto claim 7, which comprises: defining the start of the second timewindow at the end of a third time period after a transition of theair/fuel mixture from lean to rich; defining the end of the second timewindow at the end of a fourth time period after the transition of theair/fuel mixture from lean to rich; and defining the third time periodand the fourth time period as a function of a speed of the engine and avalue representing a load of the engine.
 9. The process according toclaim 7, wherein: defining the start of the second time window upon atransition of the air/fuel mixture from rich to lean; defining the endof the second time window at an end of a sixth time period after thetransition of the air/fuel mixture from rich to lean; and defining thesixth time period as a function of a speed of the engine and of a valuerepresenting a load of the engine.